Optical disc, optical disc apparatus, and digital work publication

ABSTRACT

An optical disc is based on the following the distance of the light transmission layer from a light incidence plane to the first recording layer is a minimum of 550 μm. The distance between the first and the third recording layer is a maximum of 72 μm. The distance between the second and the third recording layer is a minimum of 15 μm. The distance between the first and the second recording layer is about 31 to 40 μm. The reflectivities of the first and the second recording layer with respect to the first laser beam are 18% or more, and the ratio between the reflectivities is about 1.15 or less. The areal recording density of the third recording layer is three times or more as high as the areal recording density of the first and the second recording layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2005-237741, filed Aug. 18, 2005, theentire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

One embodiment of the invention relates to an optical disc such as a DVDwhich serves as a medium to store digitized video and audio works suchas movies and music. It further relates to an optical disc apparatuswhich reads information recorded on the optical disc, and a digital workpublication using the optical disc as a recording medium.

2. Description of the Related Art

<Outline of the DVD Standard>

One known type of optical disc for storing digital images is a DigitalVersatile Disc (DVD), which has been widely used all over the worldmainly in storing and delivering movie content (digital workpublications). This DVD is a format created by the DVD Forum, which isopen to the public as DVD Book (refer to the World Wide Web:dvdforum.org). The DVD has also been determined in internationalstandards and JIS. Here, the international standard ISO/IEC 16448 for120 mm DVD-ROM, one of the DVD physical formats, will be brieflyexplained. Moreover, there is ECMA-267 as a document associated with theinternational standards.

There are four types of 120 mm DVD-ROM: single-sided single layer,single-sided dual layer, double-sided single layer, and double-sideddual layer. In delivery of an accumulation of content of movies and thelike, two types of single-sided discs are mainly used: one is asingle-sided single layer DVD disc (4.7 GB) and the other is asingle-sided dual layer DVD disc (8.54 GB). However, recently, thesingle-sided dual layer DVD disc has accounted for 60% of the total.

On the other hand, the development of a disc whose capacity is largerthan that of the aforementioned DVD (referred to as the existing DVD)has been desired. This comes from a desire to store High Definition (HD)images into a single disc (temporarily referred to as a next-generationDVD).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A general architecture that implements the various feature of theinvention will now be described with reference to the drawings. Thedrawings and the associated descriptions are provided to illustrateembodiments of the invention and not to limit the scope of theinvention.

FIG. 1 is an exemplary diagram showing the relationship between thebasic structure of a single-sided dual layer DVD disc and an opticalhead;

FIG. 2 is an exemplary diagram showing the position of the recordinglayers of the single-sided dual layer DVD disc;

FIG. 3 is an exemplary diagram showing the relationship between thebasic structure of a single-sided dual layer HD DVD disc and an opticalhead;

FIG. 4 is an exemplary diagram showing the position of the recordinglayers of the single-sided dual layer HD DVD disc;

FIG. 5 is an exemplary diagram showing the relationship between thebasic structure of an HD DVD/DVD Twin format disc and an optical head;

FIG. 6 is an exemplary diagram showing the position of the recordinglayers in the HD DVD/DVD Twin format disc;

FIG. 7 is an exemplary diagram showing the relationship between thebasic structure of an optical disc of the present invention and anoptical head;

FIG. 8 is an exemplary diagram showing one example of how to design thethickness of substrates and interlayer thickness of the optical disc ofthe present invention;

FIGS. 9A and 9B are diagrams showing actual measurement values of thereflectivity and transmissivity of an Ag alloy film;

FIGS. 10A and 10B are diagrams showing calculated values of thereflectivity and transmissivity of the Ag alloy film;

FIG. 11 is an exemplary diagram showing the reflectivities of layers ofthe optical disc of the present invention with respect to a red laserbeam;

FIG. 12 is an exemplary diagram showing the reflectivities of the layersof the optical disc of the present invention with respect to ablue-violet laser beam;

FIG. 13 is an exemplary diagram showing the configuration of a playercomplying with the DVD standard;

FIG. 14 is an exemplary diagram showing an operation flow when theoptical disc of the present invention is played back with a red laserbeam on the player complying with the DVD standard;

FIG. 15 is an exemplary diagram showing the relationship between focussignals and a focus servo when the optical disc of the present inventionis played back with a red laser beam on the player complying with theDVD standard;

FIG. 16 is an exemplary diagram showing the configuration of an HD DVDplayer complying with the optical disc of the present invention;

FIG. 17 is an exemplary diagram showing an operation flow when theoptical disc of the present invention is played back on the HD DVDplayer complying with the optical disc of the present invention;

FIG. 18 is an exemplary diagram showing the relationship between focussignals and a focus servo when the optical disc of the present inventionis played back with a blue-violet laser beam on the HD DVD playercomplying with the optical disc of the present invention;

FIG. 19 is an exemplary diagram showing the configuration of an HDDVD/DVD compatible player complying with the optical disc of the presentinvention; and

FIG. 20 is an exemplary diagram showing an operation flow of the DVD/DVDcompatible player complying with the optical disc of the presentinvention.

DETAILED DESCRIPTION

Various embodiments according to the invention will be describedhereinafter with reference to the accompanying drawings.

If a next-generation DVD is developed, a conventional DVD device (driveor player) can not read data from the next-generation DVD since thenext-generation DVD is substantially different from the existing DVD inrecording density, modulation system, signal processing, track format,and the like. That is, the conventional DVD device has the disadvantageof being unable to read conventional DVD movie content recorded on thenext-generation DVD disc as well as high definition movie contentrecorded on the next-generation DVD disc, which may lead to a factorthat hinders the spread of the next-generation DVD.

Furthermore, since single-sided dual layer DVD discs account for 60% ofall the existing DVD discs, there is a desire for a new next-generationdisc which can be treated as if the single-sided dual layer DVD disc istreated.

Thus, in this embodiment, a single disc can deal with not onlyinformation (content) in an HD DVD but also information (content) in asingle-sided dual layer DVD.

This one embodiment is based on the configuration of a single-sidedtriple layer optical disc comprising: a light transmission layer; afirst recording layer and a second recording layer which are accessedwith a first laser beam; and a third recording layer which is accessedwith a second laser beam, these layers being arranged in that order in adirection in which the laser beams enter, wherein the distance of thelight transmission layer from a light incidence plane to the firstrecording layer is a minimum of 550 μm, the distance between the firstrecording layer and the third recording layer is a maximum of 72 μm, thedistance between the second recording layer and the third recordinglayer is a minimum of 15 μm, and the areal recording density of thethird recording layer is three times or more as high as the arealrecording density of the first recording layer and the second recordinglayer.

Hereinafter, referring to the drawings, embodiments of the presentinvention will be explained. To make it easier to understand the presentinvention, the technologies of the existing DVD and the next-generationDVD will be explained using FIGS. 1 to 6. Then, the basic configurationof a next-generation DVD according to the present invention will beexplained using FIG. 7.

<Single-Sided Dual Layer DVD>

FIG. 1 shows the relationship between the basic structure of asingle-sided dual layer DVD disc 10 and an optical head. As is wellknown, this disc has a DVD layer (L0) 15 and a DVD layer (L1) 17. Thetwo DVD layers can be accessed from one side of the disc, therebyreproducing a signal. As shown in FIG. 1, when viewed from a lightincidence plane 11, there are arranged a light transmission layer 14,the DVD layer (L0) 15, and the DVD layer (L1) 17 in that order. Theindividual DVD layers are accessed by moving an objective lens 21 underthe control of a lens actuator and performing a layer jump.

The dual layer disc is mainly characterized in that it can be producedin about the same manner as a single-sided single layer DVD disc. First,there are separately produced, by use of an injection molding machine,an L0 substrate 12 where the DVD layer (L0) 15 is formed, and an L1substrate 13 where the DVD layer (L1) 17 is formed. Next, a lowreflection film is formed to the DVD layer (L0) 15, and ahigh-reflectivity film is formed to the DVD layer (L1) 17. Then, the twosubstrates are bonded together with a space layer 16 in such a mannerthat the DVD layers lie between the two substrates, which completes thedisc. It is to be noted that a spiral direction of a track formed in theL1 substrate 13 is opposite to a spiral direction of a track formed inthe L0 substrate 12, but after the bonding, they are in the samedirection when viewed from the light incidence plane 11.

FIG. 2 is a diagram showing the position of the DVD layers when viewedfrom the light incidence plane 11 of the single-sided dual layer DVDdisc. The DVD layer (L0) 15 is limited to a position a minimum of 550 μmaway from the light incidence plane, and the DVD layer (L1) 17 islimited to a position a maximum of 640 μm away from the light incidenceplane, taking into account the spherical aberration of the objectivelens and crosstalk between the DVD layers. The distance between the twolayers (space layer 16) is set at 40 to 70 μm in terms of the layercrosstalk. The space layer 16 is generally equal to the thickness of anadhesive layer with which the two substrates are bonded together. In anactual manufacture, the distance is determined, taking into account thebonding accuracy and the formation accuracy of the L0 substrate 12. Itis to be noted that the linear recording density is reduced by 10% ascompared with a single-sided single layer DVD disc and the capacity perlayer is 4.27 GB. Jitter is regulated to be below 8.0%.

On the other hand, the reflectivities of the DVD layers are determinedas follows:

Single-layer disc: 45% to 85% (with PBS)

Dual layer disc: 18% to 30% (with PBS)

Identification information indicating the reflectivity, layer structureand the like of the disc is in Identification Data (ID) of a Data frameand in Physical Format Information (PFI) in a Control Data Zone (CDZ)located in a Lead-in area of the DVD layer (L0) 15. It is to be notedthat a Burst Cutting Area (BCA) can not be formed in a DVD video inwhich images are dealt with.

<HD DVD>

On the other hand, as has been often reported in recent years, an HD DVDhas been proposed wherein blue-violet semiconductor laser (hereinafter,referred to as blue-violet laser) is used to achieve a density threetimes or more as high as that of DVD in order to satisfy a desire tostore High Definition (HD) images onto a single disc. The HD DVD hasbeen standardized in the DVD Forum (refer to the World Wide Web:dvdforum.org). This has not been commercialized yet.).

The HD DVD has the same disc structure as that of a conventional DVD. Asingle-sided single layer HD DVD disc has a capacity of 15 GB and asingle-sided dual layer HD DVD disc has a capacity of 30 GB. These largecapacities have been realized by new technologies, including a shorterwavelength of laser light, a larger NA, a modulation system, and newsignal processing (partial response and maximum likelihood PRML).

FIG. 3 shows the relationship between the basic structure of asingle-sided dual layer HD DVD disc 30 and an optical head. In the HDDVD, a laser beam for reading information from the disc has been changedfrom a red laser beam (650 nm) to a blue-violet laser beam (405 nm) 40to have a shorter wavelength, and NA of an objective lens 41 hasincreased from 0.6 to 0.65, resulting in a different sphericalaberration and a different coma aberration caused by a tilt. Therefore,an actual disc is different from the DVD, for example, in terms of theposition of an HD DVD layer (L0) 35 and an HD DVD layer (L1) 37 from alight incidence plane 31 and in terms of the thickness of a space layer36.

FIG. 4 shows the position of the layers of the single-sided dual layerHD DVD disc when viewed from the light incidence plane 31. Since thespherical aberration has become severer as a result of making thewavelength shorter and NA larger, the HD DVD layer (L0) 35 is limited toa position a minimum of 578 μm away from the light incidence plane andthe HD DVD layer (L1) 37 is limited to a position a maximum of 622 μmaway from the light incidence plane. The distance between the two layers(space layer 36) is set to be 15 to 25 μm.

On the other hand, the reflectivities of the HD DVD layers aredetermined as follows:

Single-layer disc: 40% to 70% (including birefringence)

Dual layer disc: 18% to 32% (including birefringence)

As in a DVD, identification information indicating the reflectivity,layer structure and the like of the disc is in Identification Data (ID)of a Data frame and in Physical Format Information (PFI) in a ControlData Zone (CDZ) located in a System Lead-in area of the DVD layer (L0)35. In addition, in the HD DVD, identification information, contentprotection information and the like for the disc are provided in a BurstCutting Area (BCA) formed inside the Lead-in area. This BCA is formed inthe HD DVD layer (L1) 37.

<Existing DVD and HD DVD>

Thus, the high-capacity HD DVD capable of storing HD images has beenproposed. An HD DVD device (drive or player) newly designed for the HDDVD can be designed to be able to read data from not only an HD DVD discbut also a DVD. However, since this HD DVD disc is substantiallydifferent from the existing DVD in the recording density, modulationsystem, signal processing, track format, and the like, a conventionalDVD device (drive or player) can not read information recorded thereon.That is, the conventional DVD device has the disadvantage of beingunable to read not only the high definition movie content recorded onthe HD DVD disc but also the conventional DVD movie content. In order tocope with the problem, an HD DVD/DVD Twin format disc having an HD DVDrecording layer and a DVD recording layer has recently been standardizedin the HD DVD format (refer to the World Wide Web: dvdforum.org).

<HD DVD/DVD Twin Format Disc>

The twin format disc is a new disc which can be treated as a DVD disc inthe conventional DVD device and can be treated as an HD DVD disc in theHD DVD device. Moreover, if a device compatible with both the formats isused, this disc permits information (such as content) in both theformats to be selected by a user and read.

FIG. 5 shows the relationship between the basic structure of an HDDVD/DVD Twin format disc and an optical head. A Twin format disc 50comprises a DVD substrate 52 where a DVD layer is formed, an HD DVDsubstrate 53 where an HD DVD layer is formed, and a space layer 56. Whenviewed from a light incidence plane 51, there are formed a DVD layer 55and an HD DVD layer 57 in this order.

FIG. 6 shows the relationship of the position of the recording layers inthe Twin format disc. From the light incidence plane 51, the DVD layer55 is positioned 550 to 575 μm and the HD DVD layer 57 is positioned 578to 622 μm, while the interlayer 56 extends from 33 to 47 μmtherebetween. It is to be noted that the maximum distance between theDVD layer 55 and the HD. DVD layer 57 is 72 μm.

Furthermore, the reflectivity of this disc when read with a red laserbeam is regulated as follows:

DVD layer: 45% or more

HD DVD layer: below 8%

In the current single-sided single layer DVD, there is no definition oflayer crosstalk, but crosstalk from the HD DVD layer is regulated sothat it can be successfully read by the current DVD devices (if thelayer crosstalk is close to 8%, information can not be read by someplayers).

On the other hand, the reflectivity when reading with a blue-violetlaser beam is regulated as follows:

HD DVD layer: 14 to 28%

As just described, the twin format disc has been standardized in the HDDVD format, such that the conventional DVD device can also read the DVDinformation.

Identification information indicating the reflectivity, layer structureand the like of this disc is in the DVD layer 55 and the HD DVD layer57. Moreover, in the HD DVD layer 57, a BCA is formed inside the lead-inarea.

However, since the single layer DVD disc is defined in this twin formatdisc, there is a problem that it is not possible to record contentproduced for the dual layer DVD discs which constitute the majority ofthe actual market.

Therefore, the present inventors have devised an optical disc, anoptical disc apparatus, an optical disc reproducing method, and adigital work publication using the optical disc as a medium which enablea single disc to deal with not only the information (content) in the HDDVD but also the information (content) in the single-sided dual layerDVD. Hereinafter, specific embodiments thereof will be explained.

<Basic Configuration of an Optical Disc>

FIG. 7 shows the relationship between an optical disc according to oneembodiment of the present invention and an optical head. In an opticaldisc 70, there are formed, from a light incidence plane 71 in order, afirst signal substrate 72, a first recording layer (DVD layer (L0)) 75,a first space layer 76, a second recording layer (DVD layer (L1)) 77, asecond space layer 80 and a third recording layer (HD DVD layer (L2))81. Data is read from the DVD recording layers (L0) 75 and (L1) 77through an objective lens 21 with a red laser beam 20, and data is readfrom the HD DVD layer (L2) through an objective lens 41 with ablue-violet laser beam 40.

FIG. 8 is a diagram showing one example of how to design the thicknessof substrates and space layer thickness from the light incidence plane71 to the respective recording layers. From the light incidence plane71, the DVD layer (L0) 75 is located a minimum of 550 μm, while the HDDVD layer (L2) 81 is located a maximum of 622 μm. Thus, an allowabledistance between the two layers is a maximum of 72 μm. The threerecording layers including the DVD layers (L0) and (L1) and the HD DVDlayer (L2) can be arranged within 72 μm in such a manner as to satisfytheir standards in a range which permits practical manufacture.

In the current DVD standard, the space layer distance of the dual layerDVD disc is a minimum of 40 μm, while the space layer distance of thedual layer HD DVD disc is a minimum of 15 μm. If these distances aresubtracted from 72 μm, there remains a room of 17 μm. The accuracy ofmanufacturing the first signal substrate 72, the first space layer 76and the second space layer 80 has to be covered within 17 μm.

However, in the current manufacturing technique, the accuracy in theinjection molding of the first signal substrate can be about ±8 μm giventhe manufacturing accuracy of a stamper and attachment accuracy.Naturally, it is considered that the accuracy can be further increasedin the future. On the other hand, when the DVD recording layer (L1) 77is formed at about 30 to 40 μm on the DVD layer (L0) via the first spacelayer 76 by use of a 2P method, the current technique permits anaccuracy of about ±2 μm. Moreover, when the second space layer 80 ofabout 15 to 20 μm is used to bond the first signal substrate 72 wherethe DVD layers are formed to the HD DVD layer (L2) 81 formed on a secondsignal substrate 73, a recent vacuum bonding technique permits anaccuracy of about ±2 μm. These manufacturing tolerances when combinedadd up to 24 μm, so that there is a shortage of about 7 μm (=17 μm-24μm) considering the practical manufacture.

This is presently one of the reasons that two DVD layers can not beprovided in the HD DVD/DVD twin format disc.

It has been found out that limiting the ratio between the reflectivitiesof the DVD layers permits the space layer distance to be reduced to someextent without increasing the layer crosstalk, and the present inventionhas been made under this fact.

FIG. 8 shows one practical example of the positional relation among therecording layers of the present invention. In the present invention, theratio between the reflectivities of the DVD layers (L0) 75 and (L1) 77is limited to about 1.15 to reduce the minimum value of the first spacelayer 76 to about 33 μm. As a result, the first space layer 76 is 33 to37 μm, and the second space layer 80 is 15 to 19 μm, so that, from thelight incidence plane 71, the DVD layer (L0) 75 is located 558±8 μm, theDVD layer (L1) 77 is located 593±10 μm, and the HD DVD layer (L2) 81 islocated 610±12 μm. Thus, three recording layers can be formed in onedisc: the layers of the single-sided dual layer DVD and the HD DVD.

<Layer Crosstalk of DVD and the Space Layer>

The space layer of the DVD is set to be 40 to 70 μm, while thereflectivity thereof is 18 to 30%. This means that the regulation of thereflectivity including the space layer crosstalk is achieved even if thespace layer distance is a minimum of 40 μm. That is, the deteriorationof signals due to the crosstalk does not matter even if the layercrosstalk increases to (30%/18%)≅1.67. This means that if the ratio ofdifference in reflectivity between the DVD layers (L0) 75 and (L1) 77 islow, the range of the layer crosstalk can be the same as the range oforiginally designed values even when the space layer distance (the firstspace layer 76) is reduced correspondingly.

Now, if the reflectivities of the DVD layers (L0) 75 and (L1) 77 are inequal ratio, the space layer distance (the first space layer 76) can bereduced to40 μm/√{square root over ((1.667))}=31 μm

with the same layer crosstalk.

Now, to be more practical, if the ratio between the reflectivities ofthe DVD layers (L0) 75 and (L1) 77 is limited to 1.15 or less, the spacelayer distance (the first interlayer 76) is40 μm/√{square root over ((1.667/1.15))}=33.2 μm

so that the layer crosstalk can be the same even when the distance isreduced from 40 μm of the DVD standard to 33 μm.

On the other hand, the thickness of the space layer 36 of the dual layerHD DVD is 15 to 25 μm in the HD DVD standard. When the crosstalk has theworst value, the space layer serving as a bonding layer has a smallthickness of 15 μm, and it is thus considered that a further reductionin thickness is not so preferable in light of bonding strength.

The formation accuracy of the first signal substrate is calculated at ±8μm in the example described above, but it is presumed that the value canbe smaller than ±8 μm if the formation accuracy and the accuracy of thestamper are increased in the future. Further, it can be expected thatthe bonding accuracy and the formation accuracy by 2P are also improvedin the future. On the other hand, the reflectivities of the L0 layer andL1 layer are decided by the accuracy of a sputter device, and the ratiotherebetween can be brought as close to 1 as possible. Therefore, thespace layer distance between the L0 layer and L1 layer can be selectedfrom 31 μm to 40 μm depending on the manufacturing accuracy.

<Reflectivities of the Respective Layers>

In an actual optical disc, in addition to the position of the recordinglayers when viewed from the light incidence plane, the following isrequired: the reflectivities of the DVD layers (L0) 75 and (L1) 77 whenread with a red laser beam satisfy 18 to 30% of the standard and thedifference between the reflectivities is 1.15 or less; and the influenceof the HD DVD layer (L2) 81 on the DVD layer (L1) 77, that is, thereflectivity of the HD DVD layer (L2) with respect to the red laser beamis below 8% in the Twin format disc.

On the other hand, when the disc is read with the blue-violet laserbeam, it is required that the reflectivity of the HD DVD layer (L2) 81be 14 to 28% as in the twin format disc, that the maximum value of thereflectivity of the DVD layer (L1) 77 be 28% or less, and that the ratiobetween the reflectivities of the DVD layers (L1) 77 and the HD DVDlayer (L2) 81 be 30%/18%=1.67 or less.

In the embodiment of the present invention, a disc configuration will beshown wherein an Ag alloy film is used for the reflection film of theDVD layer and an Al alloy film is used for the reflection film of the HDDVD recording layer in order to satisfy the conditions described above.Reflection loss in the light incidence plane 71 is 10% in double passfor simplicity concerning the red laser beam and the blue-violet laserbeam. Moreover, the birefringence of the disc is 60 nm in the samedouble pass as the double pass in the HD DVD-ROM standard. The loss is amaximum of 20% for the blue-violet laser beam, and a maximum of 8.2% forthe red laser beam. The final result is considered.

FIGS. 9A and 9B show actual measurement values of the reflectivity andtransmissivity of the Ag alloy film when its thickness is changed,wherein FIG. 9A concerns the red laser beam (650 nm) and FIG. 9Bconcerns the blue-violet laser beam (405 nm). FIGS. 10A and 19B show thereflectivity and transmissivity of the Ag alloy film calculated on thebasis of the above actual measurement values, wherein FIG. 10A concernsthe red laser beam (650 nm) and FIG. 10B concerns the blue-violet laserbeam (405 nm). The calculations below were carried out using thereflectivity and transmissivity in FIG. 10.

FIG. 11 shows the relationship between light reflected from therespective layers and the reflectivity thereof when the red laser beam(Ir) 20 has entered the optical disc 70 of the present invention. Thereflectivities of the respective layers can be calculated by use of thereflectivities and transmissivities of the respective recording films.However, the loss in the incidence plane is 10%, and the effects of thebirefringence are considered later and are not included in theseequations.

The reflectivity of the DVD layer (L0) 75:Rr0=Ir0/Ir≅0.9×rr0  (1)

The reflectivity of the DVD layer (L1) 77:Rr1=Ir1/Ir≅0.9×(tr0)²×rr1  (2)

The reflectivity of the HD DVD layer (L2) 81:Rr2=Ir2/Ir≅0.9×(tr0)²×(tr1)²×rr2  (3)

In the same manner, FIG. 12 shows the relationship between lightreflected from the respective layers and the reflectivity thereof whenthe blue-violet laser beam 40 has entered the optical disc 70 of thepresent invention. The reflectivities of the respective layers can becalculated by use of the reflectivities and transmissivities of therespective recording layers. However, the loss in the incidence plane is10%, and the effects of the birefringence are considered later and arenot included in these equations.

The reflectivity of the DVD layer (L0) 75:Rb0=Ib0/Ib≅0.9×rb0  (4)

The reflectivity of the DVD layer (L1) 77:Rb1=Ib1/Ib≅0.9×(tb0)²×rb1  (5)

The reflectivity of the HD DVD layer (L2) 81:Rb2=Ib2/Ib≅0.9×(tb0)²×(tb1)²×rb2  (6)

One specific example of a design of the present invention will be shownbelow.

A calculation is made using FIGS. 10A and 10B, wherein (a) the thicknessof the Ag alloy film of the L0 layer is 9 nm, (b) the reflectivities ofthe L0 layer and the L1 layer have the same value, and (c) an Al alloyfilm is used for the reflection film of the L2 layer (here, in the Alalloy film, rr2=73.1% as to the red laser beam, and rb2=73.7% as to theblue-violet laser beam).

When the red laser beam has entered,

Rr0≅23.5% (rr0=26.1%) from Equation (1)

Rr1≅23.3% (tr0=69.3%, rr1=53.8%) from Equation (2)

Rr2≅5.2% (tr1=40.5%, rr2=73.1%) from Equation (3)

When the blue-violet laser beam has entered,

Rb0≅7.5% (rb0=8.4%) from Equation (4)

Rb1≅14.9% (tb0=85.8%, rb1=22.5%) from Equation (5)

Rb2≅22.9% (tb1=68.5%, rb2=73.7%) from Equation (6)

The results of these calculations are shown in Table 1. (a) concerns acase without birefringence, and (b) concerns a case with a maximumbirefringence of 60 nm (it decreases by 8.2% for the red laser beam anddecreases by 20% for the blue-violet laser beam). As understood fromthis table, the regulations of the dual layer DVD disc and the HD DVDdisc are satisfied. Red laser Blue-violet Recording layer beam laserbeam (a) Without birefringence DVD layer (L0) 75 23.5%  7.5% DVD layer(L1) 77 23.3% 14.9% HD DVD layer (L2) 81 5.2% 22.9% (b) With a maximumbirefringence of 60 nm DVD layer (L0) 75 21.6%   6% DVD layer (L1) 7721.4% 11.9% HD DVD layer (L2) 81 4.8% 18.3%

It is to be noted that a decrease of reflectivity due to the incidenceplane 71 of the optical disc is set at 10%, but the results in Table 1can be increased to a little less than a maximum of 10% ifanti-reflection coating or the like is employed.

<Layer Crosstalk>

It is understood from Table 1 that when the red laser beam 20 hasentered the optical disc of the present invention, the layer crosstalkfrom the HD DVD layer (L2) 81 to the DVD layer (L1) 77 is 4.8 to 5.2%,which is sufficiently lower than 8% required in the twin format disc anddoes not matter.

On the other hand, it is understood that when the blue-violet laser beam40 has entered, the layer crosstalk from the DVD layer (L1) 77 to the HDDVD layer (L2) 81 is 11.9 to 14.9%, which is lower than 18.3 to 22.9% inthe HD DVD layer (L2) 81 and does not matter.

Next, an examination is made of how the reflectivity with the red laserbeam 20 changes with the changes in the thickness of the DVD layers (L0)75 and (L1) 77. In this change of reflectivity, it is required that theratio between the reflectivities be 1.15 or less because the thicknessof the first space layer 76 is reduced from 40 nm to 33 nm.

When there is a change of ±0.3 nm (a practically larger value) in athickness of 9 nm of the Ag alloy film of the DVD layer (L0) and in athickness of 16 nm of the Ag alloy film of the DVD layer (L1), acalculation is made of how much the reflectivities of the DVD layer (L0)75 and the DVD layer (L1) 77 change.

The reflectivity and transmissivity of the DVD layer (L0) 75

8.7 nm: rr0=24.81% tr0=70.68%

9.3 nm: rr0=27.39% tr0=67.92%

The reflectivity and transmissivity of the DVD layer (L1) 77

15.7 nm: rr1=52.75% tr1=41.82%

16.3 nm: rr1=54.76% tr1=39.84%

where Rr0 and Rr1 are:

Rr0=22.33 to 24.65%

Rr1=21.9 to 24.62%

and the ratio between the reflectivities of the two recording layerssatisfies 1.126 in the worst case, and satisfies 1.15 or less. Thus, itis apparent that even when there is a change in the film thickness, theratio of the reflectivities between the two layers is 1.15 or less evenif the distance (the first space layer 76) between the DVD layers isreduced from 40 μm to 32 μm, and the layer crosstalk does not matter.

<Flag Information>

Next, a set of flags in the optical disc of the present invention willbe explained. The optical disc of the present invention has two DVDlayers, so that in the ID composed of four bytes of the Data Frame ofthe recording layers (L0) 75 and (L1) 77, bit positions b29 and b24 arewritten as

b29 (reflectivity): 1b (reflectivity is 40% or less)

b24 (layer number): 0b (in the case of the recording layer (L0))

1b (in the case of the recording layer (L1)).

b6b5 of (BP2) of the PFI in the Control Data Zone (CDZ) in the Lead-inarea formed in the DVD layer (L0) 75 indicates the number of layers inthe disc, so that

b6b5 (number of layers): 01b (two layers) is written.

Moreover, (BP16) indicates the presence of the BCA, and there must notbe the BCA in a DVD video, so that

b7 (BCA flag): 0b (without BCA) is written.

On the other hand, since the HD DVD layer (L2) 81 needs to be treated asa single layer HD DVD disc with low reflectivity, ID composed of fourbytes of the Data Frame is written as

b29 (reflectivity): 1b (reflectivity is 40% or less)

b24 (layer number): 0b (recording layer (L0)).

b6b5 of (BP2) of the PFI in the Control Data Zone (CDZ) in the Lead-inarea formed in the HD DVD layer (L2) 81 indicates the number of layersin the disc:

b6b5 (Number of layers): 00b (one layer) is written.

(BP16) indicates the presence of the BCA, so that

b7 (BCA flag): 1b (with BCA) is written.

Furthermore, in the HD DVD/DVD Twin format disc, there are regulationsof the twin format disc for the conventional single layer DVD and singlelayer HD DVD, so that (BP33) includes

Layer 0 (b5-b3): 100b (DVD-ROM format)

Layer 1 (b2-b0): 000b (HD DVD-ROM format).

Note that since the disc of the present invention has a triple-layerstructure, this regulation for the dual layer structure is not applied.However, it can be judged that there are two DVD layers if the DVDlayers are accessed, and it is therefore possible to use the regulationas it is.

However, it is truly more convenient if it can be recognized that thereare two DVD layers when the HD DVD layer is accessed. In this case, forexample, 101b (dual layer DVD-ROM format) has only to be newly definedin layer 0 (b5-b3).

Next, flag information of the BCA will be explained. A BCA record of theHD DVD has 8 bytes, and (BP4) indicates a book type and a disc type.Since a Twin format flag indicating the twin format disc is in b2therein,

b2 (Twin format flag): 1b (Twin format disc) is written.

<Reproduction by an Optical Disc Apparatus Complying with the DVDStandard>

Next, a case where the optical disc of the present invention is playedback on a conventional DVD player will be explained using FIGS. 13, 14and 15. FIG. 13 shows the main configuration of a well-knownconventional DVD player. FIG. 14 is a flowchart to help explain theoperation of the DVD player. FIG. 15 shows focus signals and a focusservo.

A spindle motor 100 rotates/drives a turntable. A damper 101 holds theoptical disc 70 onto the turntable. The spindle motor 100 is controlledby a motor driver 102. An optical head 110 includes the objective lens21 and an optical system 113. The optical system 113 is driven by afocus and tracking actuator 116. When the focus and tracking actuator116 is controlled by an actuator driver 118, the laser beam is focusedon a track on the optical disc and follows the track. A radial actuator117 is used to move the optical head 110 in the direction of radius ofthe disc and is controlled by the actuator driver 118.

The reflected light from the disc is taken out of the optical system 113and is converted into an electric signal at a photodetector in aconversion unit 115. The electric signal is gain-adjusted at areproduced signal amplifier in a gain adjusting unit 120 and theresulting signal is input to a signal processing circuit 130. The signalprocessing circuit 130 performs a demodulating process, buffering, errorcorrection, and others and inputs the resulting signal to a dataprocessing circuit 140. Here, the data processing circuit 140 performspacket separation, control signal separation, and the like and inputsvideo and audio information to an AV decoder 150. The video signal,audio signal, sub-video signal, and the like demodulated at the AVdecoder 150 are output as a baseband signal via an AV amplifier 160, andinput to a monitor.

Using a focus error signal and tracking error signal obtained by, forexample, numerically processing the reproduced signal from a 4-quadrantphotodiode, a servo controller 170 supplies a control signal to theactuator driver 118. In response to a signal from an input terminal(e.g., a remote controller or an operation key input section) 190, asystem controller 180 controls the playback, stop, and temporary stop ofthe apparatus, and the like. In addition, the system controller 180controls a laser diode driver in the gain adjusting unit 120. The laserdiode driver drives the laser diode installed in the optical head 110,thereby outputting a laser beam.

When the optical disc 70 of the present invention is inserted in the DVDplayer, the spindle motor 100 is rotated until a specific number ofrevolutions has been reached (in step 200 in FIG. 14). Next, the redlaser beam 20 is turned on, and a periodic driving current is caused toflow through the focus actuator (ACT) 116, thereby moving the opticalhead up and down in the direction of axis (in steps 201 and 202 in FIG.14). A focus signal 203 produced from the reproduced signal periodicallyappears (see FIG. 15). The lowest reflectivity of the dual layer DVD is18%, so that if an FS detection level 204 of about 9% is set, twodetection pulses 207 are obtained in a focus detection signal 206 (insteps 205 and 207 in FIG. 14).

From the fact that the two pulses have been detected, this disc isjudged to be a dual layer DVD (an actual judgment is made by thePhysical Format Information (PFI) described later), thereby entering asequence to play back the dual layer DVD (in step 208 in FIG. 14).

It is to be noted that the FS detection level 204 is set at about 7%lower than 9% in some DVD players. Even in that case, the optical discof the present invention is not erroneously detected as a triple layerdisc because the reflectivity of the HD DVD layer is 5.3% or less.

After a gain adjustment 210 of the reproduced signal is first made (instep 210 in FIG. 14), the DVD layer (L0) 75 is focused on (in step 214in FIG. 14). After a short stabilization time has elapsed, the recordinglayer (L0) 75 is in focused-on state (in step 204 in FIG. 14).

Then, when a suitable position of the disc is tracked on (in step 220 inFIG. 14), the reproduced signal can be read. The optical head 110 islocated at a given position of the disc, and reads the ID of the dataframe of the reproduced signal, thereby making it possible to judgewhether the recording layer is the recording layer (L0) or (L1) andwhether the position is in a data area or in the Lead-in area, and alsoto know the set reflectivity (in step 221 in FIG. 14).

Next, the radial ACT 117 is driven, and the optical head is moved to thelead-in area (in step 222 in FIG. 14), and then the PFI in the ControlData Zone (CDZ) is read (in step 223 in FIG. 14). Here, (BP2) is checkedto verify that the disc is a dual layer DVD disc, which is followed bythe reproduction of dual layer DVD images (in step 226 in FIG. 14). Theuser can enjoy the DVD images using the input terminal 190.

<Reproduction by an Optical Disc Apparatus Complying with the HD DVDStandard>

Next, a case of an HD DVD player using the blue-violet laser beam willbe explained using FIGS. 16, 17 and 18. FIG. 16 shows the mainconfiguration of the HD DVD player. FIG. 17 is a flowchart to helpexplain the operation of the HD DVD player. FIG. 18 shows focus signalsand a focus servo. The configuration of the HD DVD player is similar tothat of the apparatus shown in FIG. 13, and the same numerals aretherefore given to similar parts.

When the optical disc 70 of the present invention is inserted in the HDDVD apparatus, a spindle motor 100 is rotated until a specific number ofrevolutions has been reached (in step 200 in FIG. 17). Next, theblue-violet laser beam 40 is turned on (in step 231 in FIG. 17), and aperiodic driving current is caused to flow through a focus actuator ACT116, thereby moving an optical head up and down in the direction of axis(in step 232 in FIG. 17). A focus signal 233 produced from a reproducedsignal periodically appears (see FIG. 18).

The lowest reflectivity of the dual layer HD DVD is 18% which is similarto that of the dual layer DVD, so that if an FS detection level 234 isset at about 9%, two detection pulses 237 are obtained in a focusdetection signal 236. Moreover, three pulses can sometimes be obtained(in steps 235 and 237 in FIG. 17). Therefore, this disc is temporarilyjudged to be a dual layer HD DVD disc, a Twin format disc (TFD) or anoptical disc of the present invention (in step 238 in FIG. 17), therebyentering a sequence to play back the disc of the present invention (afinal judgment is made in accordance with the PFI and the flaginformation of BCA).

After a gain adjustment of the reproduced signal is first made (in step240 in FIG. 17), the HD DVD layer (L2) 81 is focused on (in step 244 inFIG. 17). After a short stabilization time has elapsed, the layer (L2)81 is in focused-on state (in step 245 in FIG. 17). Then, when asuitable position of the disc is tracked on (in step 250 in FIG. 24),the reproduced signal can be read. The optical head 110 is located at agiven position of the disc, and reads the ID of the Data frame of thereproduced signal, thereby making it possible to judge whether the layeris the recording layer (L0) 35 or (L1) 37 of the dual layer HD DVD or asingle layer (SL) and whether the position is in the data area or in theLead-in area, and also to know the set reflectivity (in step 251 in FIG.17).

If, here, the layer is identified as the (L1) instead of the (L0) or(SL), this disc is a dual layer HD DVD, thereby moving to a flow 260(not shown here) of the dual layer HD DVD.

When the disc is judged to be the L0 or SL, the radial ACT 117 isdriven, and the optical head is moved to the System Lead-in area (instep 252 in FIG. 17), and then the PFI in the Control Data Zone (CDZ) isread (in step 253 in FIG. 17). Here, it can be recognized from (BP2)that the disc is an HD DVD single layer disc, from (BP16) that the dischas a BCA, and from (BP33) that a DVD layer is formed in this disc.Subsequently, when the BCA is read (in step 254 in FIG. 17), it isfinally verified from the (BP4) of BCA record ID that the disc is anoptical disc of the present invention (in step 255 in FIG. 17), which isfollowed by the reproduction image on of HD DVD layer (in step 256 inFIG. 17). The user can enjoy the HD DVD images using an input terminal190.

<Reproduction by an Optical Disc Apparatus Complying with Both the HDDVD Standard and the DVD Standard>

Next, a compatible player according to the present invention using boththe blue-violet laser beam and the red laser beam will be explainedusing FIGS. 19 and 20. FIG. 19 shows the configuration of the compatibleplayer. FIG. 20 is a flowchart to help explain the operation of thecompatible player. The compatible player can selectively output the redlaser beam and the blue-violet laser beam. The parts similar to those inthe previously described players are given the same numerals.

First, a disc of the present invention is inserted in the compatibleplayer, and the disc is rotated (in step 200 in FIG. 20). Next, thelaser is turned on to move on to focusing/tracking, signal reproductionand image reproduction, wherein the blue-violet laser beam is firstturned on (in step 231 in FIG. 20) to reproduce the images in the HD DVDlayer.

The reproduction flow for the HD DVD images is the same as that in FIG.17. The HD DVD layer (L2) 81 is focused on (in step 244 in FIG. 20).Then, the disc is tracked on (in step 250 in FIG. 20). The Data FrameID, the PFI of the System Lead-in area, and the BCA are read, therebyjudging that this disc is an optical disc of the present invention andreproducing the images of the HD DVD. The user can enjoy the HD DVDimages using an input terminal 190.

Next, when the user specifies the playback of the DVD by use of theinput terminal 190, the red laser is turned on by a signal 261 to switchto the DVD (in step 201 in FIG. 20). This is followed by the sameoperation flow as that in FIG. 14, so that a focus actuator ACT 116 isdriven, the recording layer is searched (in step 202 in FIG. 20), theDVD layer (L0) 75 is focused on, and the disc is tracked on (in step 220in FIG. 20), thereby reproducing the dual layer DVD images (in step 226in FIG. 20).

Next, when the user again specifies the playback of the HD DVD by use ofthe input terminal 190, the blue-violet laser is turned on by a signal262 to switch to the HD DVD playback (in step 231 in FIG. 20), such thatthe HD DVD images can be reproduced in a manner previously shown (instep 256 in FIG. 20).

As described above, according to the present invention, the HD DVD andthe dual layer DVD can be formed in one optical disc. In the existingDVD apparatus, the dual layer DVD layer is played back. In the HD DVDapparatus compatible with the HD DVD standard, the HD DVD layer isplayed back. In the compatible apparatus according to the presentinvention, both the DVD layer and the HD DVD layer can be played back.It is thus possible to allow compatibility between a group of productsof the existing DVD standard and a group of products of the new HD DVDstandard, and also to accelerate a smooth spread of the HD DVD standardproduct group to general users.

Other Embodiments

In the embodiment described above, the translucent films of the firstrecording layer and the second recording layer are formed of an Agalloy. However, if reflectivity and transmissivity can be selectivelyset for each of the two laser beams different in wavelength, theapparatus can be more efficiently operated.

For example, the first recording layer and the second recording layercan be formed of multiple interference films to set a more effectivereflectivity.

Furthermore, all the recording layers described are playback-only (ROM)recording layers in the present invention, but the combination of therecording layers is not limited thereto. For example, the thirdrecording layer may be a recordable type. In that case, the reflectivityof the third recording layer is reduced to less than half of that in thecase of the ROM.

<Summary of Basic Points in the Embodiment Described Above>

An optical disc according to the present invention is basicallyspecified by the following items (1) to (7).

(1) The optical disc is a single-sided triple layer optical disc where alight transmission layer, a first recording layer and a second recordinglayer which are accessed with a first laser beam, and a third recordinglayer which is accessed with a second laser beam are arranged in thatorder in the direction in which the laser beam enters.

(2) The distance of the light transmission layer from a light incidenceplane to the first recording layer is a minimum of 550 μm.

(3) The distance between the first recording layer and the thirdrecording layer is a maximum of 72 μm.

(4) The distance between the second recording layer and the thirdrecording layer is a minimum of 15 μm.

(5) The distance between the first recording layer and the secondrecording layer is about 31 to 40 μm.

(6) The reflectivities of the first recording layer and the secondrecording layer with respect to the first laser beam are 18% or more,and the ratio between the reflectivities is about 1.15 or less.

(7) The areal recording density of the third recording layer is threetimes or more as high as the areal recording density of the firstrecording layer and the second recording layer.

Furthermore, the optical disc according to the present invention canadditionally implement the following item (8) on the basis of the aboveitems:

(8) The reflectivity of the third recording layer with respect to thesecond laser beam is 14 to 28%.

An optical disc apparatus according to the present invention isspecified by the following items (9) to (15):

(9) The optical disc apparatus is an apparatus which reads informationrecorded on an optical disc.

(10) The optical disc is a single-sided triple layer optical disc wherea light transmission layer, a first recording layer and a secondrecording layer which are accessed with a first laser beam, and a thirdrecording layer which is accessed with a second laser beam are arrangedin that order in the direction in which the laser beam enters.

(11) The distance of the light transmission layer from a light incidenceplane to the first recording layer is a minimum of 550 μm.

(12) The distance between the first recording layer and the thirdrecording layer is a maximum of 72 μm.

(13) The distance between the second recording layer and the thirdrecording layer is a minimum of 15 μm.

(14) The distance between the first recording layer and the secondrecording layer is about 31 to 40 μm.

(15) The reflectivities of the first recording layer and the secondrecording layer with respect to the first laser beam are 18% or more,and the ratio between the reflectivities is about 1.15 or less.

(16) The areal recording density of the third recording layer is threetimes or more as high as the areal recording density of the firstrecording layer and the second recording layer.

(17) The information reading apparatus comprises an optical head whichcan generate the first laser beam and the second laser beam, and controlmeans for causing the first laser beam or the second laser beam to beselectively generated.

Furthermore, the optical disc apparatus according to the presentinvention can additionally implement the following item (18) on thebasis of the above items:

(18) The reflectivity of the third recording layer with respect to thesecond laser beam is 14 to 28%.

Furthermore, the optical disc apparatus according to the presentinvention can additionally implement the following item (19) on thebasis of the above items:

(19) When the optical disc is inserted, the second laser beam is firstturned on to access the third recording layer.

Furthermore, the optical disc apparatus according to the presentinvention can additionally implement the following item (20) on thebasis of the above items:

(20) The control means selects the first laser beam or the second laserbeam in accordance with a user input from a user interface.

It is to be noted that the present invention is not limited to theembodiment described above without modification, and some of all thecomponents shown in the embodiment may be eliminated. Moreover,components in different embodiments may be suitably combined.

According to the embodiment described above, it is possible to providean optical disc which enables a first recording layer and a secondrecording layer (corresponding to a dual layer DVD layer) to be accessedwith a first laser beam (red laser beam) and enables a third recordinglayer (corresponding to an HD DVD layer) to be accessed with a secondlaser beam (blue-violet laser beam) from one side. Therefore, a singleoptical disc can contain both movie content for the currently widespreaddual layer DVD disc, and movie content for the HD DVD. As a result, itis possible to record on this optical disc most of the movie content(generally consisting of a feature film and bonus content) which havealready been provided or which will be provided in DVDs, therebyproviding a real combination disc which can deal with both standard (SD)images and high definition (HD) images.

Furthermore, DVD content can be reproduced in conventional DVDcompatible optical disc apparatus. On the other hand, the new HD DVDcompatible optical disc apparatus can be adapted to be able to reproducethe movie content for the HD DVD or to reproduce both the movie contentfor the HD DVD and the movie content for the DVD.

For example, identical movie contents are prepared as DVD content and HDDVD content, and if these two movie contents are recorded on a singledisc, a user who has a DVD compatible apparatus alone can see the DVDmovie content, while a user who has an HD DVD compatible apparatus cansee the HD DVD movie content.

If the user who does not presently have an HD DVD compatible apparatuspurchases an HD DVD compatible apparatus in the future, he/she can enjoythe HD images with an already purchased optical disc without newlypurchasing an HD DVD disc, which is a great advantage to the user.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the inventions. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the inventions. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the inventions.

1. A single-sided triple layer optical disc comprising: a lighttransmission layer; a first recording layer and a second recording layerwhich are accessed with a first laser beam; and a third recording layerwhich is accessed with a second laser beam, these layers being arrangedin that order in a direction in which the laser beams enter, wherein thedistance of the light transmission layer from a light incidence plane tothe first recording layer is a minimum of 550 μm, the distance betweenthe first recording layer and the third recording layer is a maximum of72 μm, the distance between the second recording layer and the thirdrecording layer is a minimum of 15 μm, the distance between the firstrecording layer and the second recording layer is about 31 to 40 μm, thereflectivities of the first recording layer and the second recordinglayer with respect to the first laser beam are 18% or more, the ratiobetween the reflectivities being about 1.15 or less, and the arealrecording density of the third recording layer is three times or more ashigh as the areal recording density of the first recording layer and thesecond recording layer.
 2. The optical disc according to claim 1,wherein the reflectivity of the third recording layer with respect tothe second laser beam is 14 to 28%.
 3. An optical disc apparatus whichreads information recorded on an optical disc, the optical disc being asingle-sided triple layer optical disc comprising: a light transmissionlayer; a first recording layer and a second recording layer which areaccessed with a first laser beam; and a third recording layer which isaccessed with a second laser beam, these layers being arranged in thatorder in a direction in which the laser beams enter, wherein thedistance of the light transmission layer from a light incidence plane tothe first recording layer is a minimum of 550 μm, the distance betweenthe first recording layer and the third recording layer is a maximum of72 μm, the distance between the second recording layer and the thirdrecording layer is a minimum of 15 μm, the distance between the firstrecording layer and the second recording layer is about 31 to 40 μm, thereflectivities of the first recording layer and the second recordinglayer with respect to the first laser beam are 18% or more, the ratiobetween the reflectivities being about 1.15 or less, and the arealrecording density of the third recording layer is three times or more ashigh as the areal recording density of the first recording layer and thesecond recording layer, the information reading apparatus comprising: anoptical head which is configured to generate the first laser beam andthe second laser beam; and control means for causing the first laserbeam or the second laser beam to be selectively generated.
 4. An opticaldisc apparatus which reads information recorded on an optical disc, theoptical disc being a single-sided triple layer optical disc comprising:a light transmission layer; a first recording layer and a secondrecording layer which are accessed with a first laser beam; and a thirdrecording layer which is accessed with a second laser beam, these layersbeing arranged in that order in a direction in which the laser beamsenter, wherein the distance of the light transmission layer from a lightincidence plane to the first recording layer is a minimum of 550 μm, thedistance between the first recording layer and the third recording layeris a maximum of 72 μm, the distance between the second recording layerand the third recording layer is a minimum of 15 μm, the areal recordingdensity of the third recording layer is three times or more as high asthe areal recording density of the first recording layer and the secondrecording layer, the distance between the first recording layer and thesecond recording layer is about 31 to 40 μm, the reflectivities of thefirst recording layer and the second recording layer with respect to thefirst laser beam are 18% or more, the ratio between the reflectivitiesbeing about 1.15 or less, and the reflectivity of the third recordinglayer with respect to the second laser beam is 14 to 28%, theinformation reading apparatus comprising: an optical head which isconfigured to generate the first laser beam and the second laser beam;and control means for causing the first laser beam or the second laserbeam to be selectively generated.
 5. The optical disc apparatusaccording to claim 3, wherein when the optical disc is inserted, thesecond laser beam is first turned on to access the third recordinglayer.
 6. The optical disc apparatus according to claim 4, wherein whenthe optical disc is inserted, the second laser beam is first turned onto access the third recording layer.
 7. The optical disc apparatusaccording to claim 3, wherein the control means selects the first laserbeam or the second laser beam in accordance with a user input by a userinterface.
 8. The optical disc apparatus according to claim 4, whereinthe control means selects the first laser beam or the second laser beamin accordance with a user input by a user interface.